Increase body width/decrease sidebar width

I'm very unfamiliar with code, but I'm trying to figure out if I can easily increase the width of the body of my posts, while also decreasing the sidebar width just a little. My site: http://realfitrealfoodmom.com/ You can see that the leaderboard size ad that I have at the bottom of all my blog posts is being cut off on the right hand side. I like the shape/size of that ad placement in that spot...but I don't like that it's being cut off. I assume there is an easy way that I can increase/decrease the widths of my template (body and sidebar) to make it fit? Thanks so much!